bug: .distinct(on="does_not_exist") does not crash

[NickCrews]

What happened?

ibis.examples.topo.fetch().distinct(on="does_not_exist")

I would expect this to error.

What version of ibis are you using?

5.1.0, have not tested on main

What backend(s) are you using, if any?

duckdb, but probably irrelevant?

Relevant log output

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[mesejo]

I think the problem is in the GroupedTable constructor, it allows self.by to be empty:

    def __init__(
        self, table, by, having=None, order_by=None, window=None, **expressions
    ):
        self.table = table
        self.by = list(
            itertools.chain(
                itertools.chain.from_iterable(
                    _get_group_by_key(table, v) for v in util.promote_list(by)
                ),
                (
                    expr.name(k)
                    for k, v in expressions.items()
                    for expr in _get_group_by_key(table, v)
                ),
            )
        )
        self._order_by = order_by or []
        self._having = having or []
        self._window = window